BAGHDAD, Iraq (CNN) -- A man in a wheelchair blew himself up Monday in a northern Iraqi police station, killing three National Police officers, including a commander, police said.

Soldiers stand by as pilgrims gather Sunday in Karbala for al-Arbaeen, one of holiest days on the Shiite calendar.

The attack also wounded nine officers on the police force, which the Iraqi Interior Ministry operates.

The bombing in Samarra raises concern about the recent tactics employed by insurgents in Iraq. Bombs have been placed inside dead animals and hidden in carts. And in recent days, vagrants have been involved in bombings.

"As a sign of desperation, some of those terrorists resorted to some new methods and techniques," said Maj. Gen. Qassim Atta, spokesman for Baghdad's security plan.

One of the tactics is the use of remote-controlled "sticky" bombs, small enough to tape under vehicles.

A high-ranking Samarra police official said the disabled man came to meet with Brig. Gen. Abdul Jabbar Rabei Muttar, deputy commander of security, at the security operations building in Samarra. The pair met last week as well.

The man was searched when he entered the building, but police didn't look under his wheelchair seat, where the explosives had been placed. The man, who police say was cogent, detonated the explosives when Muttar approached him.

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Also Monday, a roadside bomb exploded in the middle of a crowd of Shiite Muslims in Baghdad, killing three people and wounding 15, an Interior Ministry official said.

The strike, in southeastern Baghdad's Zafaraniya district, is the latest attack against pilgrims trekking to Karbala for al-Arbaeen, one of the holiest days of the Shiite religious calendar. It falls on Wednesday.

On Sunday, a suicide bomber in Iskandariya killed at least 45 people and wounded 68 others, and armed militants attacked pilgrims in southern Baghdad, killing three and wounding more than 30 others.

Pilgrims traditionally make their way to Karbala on foot as a demonstration of piety, and those who head to the city from Baghdad go through Babil province, where Iskandariya is located.

Joint forces have stepped up patrols during the pilgrimage to protect the thousands headed to Karbala. Last year, more than 180 pilgrims were killed in a series of attacks, most during twin suicide bombings in Hilla, Babil province's capital.

Sunday's attack in Iskandariya, conducted by a bomber wearing an explosive vest, prompted authorities there to replace the police chief, Hilla police said.

More National Police officers have been deployed to Babil to ensure the safety of pilgrims heading to Karbala, a police official said.

The U.S. Embassy in Baghdad and American-led coalition forces condemned Sunday's "barbaric attacks in Baghdad and Iskandariya" against "innocent citizens participating in an important religious commemoration."

Every year, thousands of pilgrims amass in Karbala for al-Arbaeen, which commemorates the end of a 40-day mourning period for Imam Hussein, the grandson of the Prophet Mohammed.

Hussein, one of the most revered Shiite martyrs, is buried in Karbala, about 60 miles southwest of Baghdad. Karbala's main holy site is the gold-domed Imam Hussein Shrine.

Other developments

Gunmen ambushed an Iraqi Army convoy late Monday, killing eight soldiers, an official with Baquba police told CNN Monday. The gunmen used rocket-propelled grenades and small arms fire in the attack in Buhriz, about 4 miles (7 km) south of Baquba. Baquba is about 37 miles (60 km) north of Baghdad.

A suicide bomber blew up outside a mosque south of Falluja, killing two Iraqi police officers, two civilians and the bomber, a U.S. military statement released Monday said. Twelve civilians also were wounded. The Friday attack outside the Rahman mosque apparently was carried out by an adolescent, the military said.

The military reported raids Sunday and Monday in central and northern Iraq targeting al Qaeda in Iraq. One person was killed and 11 others were detained.

The mayors of Baghdad and Tehran, Iran, signed an agreement expanding the cooperation between the cities on security and urban development matters. The Monday memorandum of understanding said the cities will cooperate on training, security and the "implementation of infrastructure projects in both cities."